there are a couple of things which could be (probably are)going on here. originally 'another' was two words (hi ot): an other, sometimes found in the literature as a nother (thru metanalysis). [Shakespeare gets some credit for combining the words.] so there is quite an atural split. the other possible way to look at this is as an example of tmesis, the separation of parts of a word by inserting a-whole-nother word, as in abso-freaking-lutely. I guaran-dam-tee it.